Can I choose or modify my seat when checking in online? Yes, you can check seat availability and select the most suitable seat when using Volaris' online check-in. Depending on the route and the kind of seat Premium, More Space, First-off, or Standard—the cost of each seat is per passenger for a single trip (taxes included). The cost of a seat can be seen under Additional Services Fees.
Link : https://www.flyamericans.com/blog/volaris-airlines-seat-upgarde